The present invention relates to a lifting vehicle, in particular a forklift truck, comprising a mobile chassis with a mast, wherein a lifting device is connected for axial movement to the mast and is coupled to a hydraulic lifting cylinder in order to perform a substantially vertical displacement, and wherein alerting means are provided for the purpose of monitoring a load on the lifting device as a result of a cargo being carried thereby.
Lifting vehicles are applied in diverse embodiments in logistics applications, often referred to simply as material handling, for the purpose of moving goods quickly and efficiently. Mainly applied here as lifting vehicle for heavier cargoes are forklift trucks with which cargoes up to several tens of tons can be lifted and displaced with a lifting device provided therein. The lifting vehicle comprises a sufficient counterweight to enable such a cargo to be carried. It is however extremely important here that the cargo will not exceed a specific maximum prescribed for the vehicle. This is expressed in a cargo chart on the vehicle which allows a maximum load depending on a lifting height, a cargo weight and a cargo position, or cargo moment.
For safe operation of a lifting vehicle such as a forklift truck it is necessary to remain within the limits of this cargo chart. For this purpose the vehicle can have been or be fitted with electronic alerting means for monitoring imminent overload of the lifting device as a result of a cargo being carried thereby. These alerting means normally comprise for this purpose one or more weight sensors for generating in real-time to the driver an indication of the weight of a cargo on the lifting device. These are for instance strain-sensitive sensors which record a mechanical deformation in or at the lifting device as a result of the cargo.
This causes a (tensile) stress which affects the strain-sensitive sensor and is generated as electronic signal. The sensor is coupled to a processing unit which is able to compare an output signal from the sensor to an optionally linear output characteristic provided therein for the purpose of deriving therefrom an indication of the actual weight of the load. The processing unit passes this indication to a monitor and/or loudspeaker provided close to the driver in order to alert the driver when a maximum allowable weight threatens to be exceeded.
A problem which occurs in practice in some cases with such alerting means is that the strain sensors applied, depending on the temperature, assembly tolerances and the operating time, display a certain drift and can as a result generate an increasingly less accurate value after a period of time. In order to overcome this an electronic calibration has to be performed periodically, whereby the output signal from sensors is reset to the associated output characteristic with which the processing unit calculates. Only then can a sufficiently accurate monitoring of the load on the vehicle be guaranteed. Such a periodic calibration does however require the necessary attention, time and discipline of a vehicle controller, and these may be disregarded.
The present invention has for its object, among others, to provide a lifting vehicle which obviates the above outlined problem to at least significant extent.